Maintain 'page' parameter for block from subscribers list, block & make-admin from...

Refactored some of the returnto handling code. It looks like we have several different ways of handling this in the software, icky!
Marked the session-based functions with fixmes (they'll stomp on other forms when multiple tabs/windows are used) and combined some commonish bits of code between ProfileFormAction and the group block & makeadmin actions where they're using hidden form parameters. Extended that to allow passing dynamic parameters (eg 'page') as well as static ones (action, target user/group).